Electrolytic process for preparation of chlorine pentafluoride



'July 8, 1969 E. A. LAWTON ETAL 3,454,476

ELECTROLYTIC PROCESS FOR PREPARATION OF CHLORINE PENTAFLUORIDE FiledJuly 12, 1963 INVENTORS EMIL A. LAWTON ,BY HOWARD H. ROGERS ATTORNEYUnited States Patent Ofice 3,454,476 Patented July 8, 1969 3,454,476ELECTROLYTIC PROCESS FOR PREPARATION OF CHLORINE PENTAFLUORIDE Emil A.Lawton and Howard H. Rogers, Woodland Hills,

Calif., assignors to North American Rockwell Corporation, a corporationof Delaware Filed July 12, 1963, Ser. No. 294,765 Int. Cl. B01k 1/00;C06b 15/00; C06d /02 US. Cl. 20459 4 Claims This invention relates to anelectrolytic process for preparation of chlorine pentafluoride (C11 Amethod for the preparation of ClF by subjecting a mixture of fluorineand chlorine, for example, to a glow discharge is described in patentapplication Ser. No. 253,521, filed Jan. 21, 1963, by Walter Maya andHans F. Bauer, now Patent 3,354,626. As therein mentioned, ClF is anextremely high-energy oxidizer of greater oxidizing potential thanchlorine trifluoride which finds util- 1ty as an oxidizer for rocketpropellant fuels. The boiling point of ClF is about 14 C., and thecompound is stable to at least 300 C. in containers of stainless steel,for example.

By the above-mentioned glow discharge process, the yield of ClF isrelatively low, whereas for the process of the instant invention theyields are substantially higher and more precisely predictable.Significantly the process of this invention involves chemical changeswhich occur with the reactants being in liquid phase whereas thereactions of the glow discharge process occur only in gaseous phase atlow pressure, and as a consequence the 6thciency of the process of thisinvention with respect to apparatus and process operations issubstantially greater than that of the glow discharge process.

Broadly stated, the invention comprises subjecting a solution ofchlorine trifluoride in hydrogen fluoride and a conductivity additive,to an electric current in an electrolytic cell, and collecting thechlorine pentafluoride which is evolved from the cell. The chemicalchanges which occur at the anode may be represented by the followingequations:

The chemical changes which occur at the cathode are apparently asfollows:

The total chemical change for the cell may be represented by theequations:

2ClF +electrical energy ClF +ClF (5 5ClF +electrical energy 3ClF +Cl-(6) Thus, in addition to the chlorine pentafluoride, other resultants ofthe electrolysis are C1 ClF, and possibly F and when impure reactantsare employed various contaminants are produced, such as C10 CIO F, ClOF, and SP The chlorine pentafluoride may be separated from the otherresultants and from the contaminants by conventional proceduresincluding vacuum fractionation.

With respect to the conductivity additive, any of the alkali metalhalides are usable, preferably an alkali metal fluoride. Pure hydrogenfluoride being practically nondissociated, the conductivity additivefurnishes the ions by which the electric current is carried through theelectrolytic cell; however, the additive is not consumed in the overallprocess, the additive being continually regenerated by the fluorine ofthe chlorine trifluoride. In cases where the additive is a halide otherthan fluorine, mere substitution of the non-fluorine halogen by fluorineatoms from the hydrogen fluoride occurs with evolution of thecorresponding hydrogen halide. It is to be noted further that thehydrogen fluoride is not consumed in the formation of the pentafluoride.

The invention is hereinafter illustrated by description with referenceto the accompanying drawing, the single figure of which is adiagrammatic representation of a suitable laboratory apparatus for theelectrochemical synthesis of chlorine pentafluoride according to theprocess of this invention.

In the drawing, reference numeral 10 designates an electrolytic cell ofstainless steel comprising a doublewalled tank 12 having acircumferentially continuous flange 13 at its top, a cover 15 secured tothe flange with a gasket 16 of Teflon between the cover and the flange.The tank has a drain 18 for emptying the cell and has an inlet 19 and anoutlet 20 for continuous flow of a coolant through the space between thetank walls. Spaced apart within the tank by about one-half inch are twoplate electrodes 22 and 23 (50 sq. cm. on each face) of a metal, e.g.nickel, which is not easily soluble in the reactants and does not forman insulating anodic film. The electrodes are suspended from the cover15 by posts 25 electrically connected by leads 27 to a power source at29, e.g. a continuously variable, full-wave rectified system includingcalibrated meters for measurement of the power.

An inert gas, e.g. helium, is preferably employed for purging theapparatus and to serve as a carrier for the product. There is a valvecontrolled line 32 extending through the cover of cell 10 and adapted tobe connect d at its outer end to a cylinder (not shown) of helium underpressure. A gauge 33 connected to the line 32 indicates the pressure inthe cell. For supplying the reactant, chlorine trifluoride, a flow tube35, having a metering section 36, is connected to the cell 10 throughthe cell cover and is adapted at its outer end for connection to acylinder (not shown) of liquid ClF under its own vapor pressure. Avalved vent 38 on the flow tube 35 permits removal of air from the tube35. For supplying hydrogen fluoride to the cell, there is avalve-controlled inlet tube 40 adapted to be connected at its upstreamend to a supply of liquid hydrogen fluoride and at its downstream end tothe inside of the electrolytic cell. A branch 41 on the line 40 servesto admit a solution of the conductivity additive in hydrogen fluoride tothe cell.

Standing upright from the center of the cell 10 is a condenser 44 withits lower end extending through the cell cover. The upper end of thecondenser is connected by a tube 45, controlled by a valve 46, to oneend of an absorber column 48 filled with sodium fluoride pellets forabsorbing any hydrogen fluoride gas which may be carried over from thecondenser 44. A by-pass 49 controlled by a valve 50 is connected to thetube 45 upstream of the valve 46 for preliminary removal of gases. Atrain of U-tube traps is connected to the downstream end of the HFabsorber 48, such train comprising a flow line 53, first and secondU-tube traps 54 and 55 of PEP- Teflon, and a flow line 56 leading to aplace for storage at 57 for the chlorine pentafluoride. A gauge 59connected to the line 56 indicates the pressure in the train of traps. Apurge outlet 60 is connected adjacent the downstream end of line 56. Thevalves of the apparatus are formed of Monel metal and except whereotherwise explained above, the rest of the apparatus is formed ofstainless steel.

In operation, the apparatus is preferably first flushed by flowinghelium from the inlet 32 to the purge outlet 60. Hydrogen fluoride andthe conductivity additive, e.g. sodium fluoride, are then introducedinto the cell to a level covering the plate portions of the electrodes.It is preferred to purify the hydrogen fluoride, if not purified whenintroduced, and for that purpose electric-current is passed between theelectrodes 22 and 23, the valve 46 is closed and the by-pass 49 isopened thereby to remove contaminants, e.g. sulfur and oxygen compoundsfrom the hydrogen fluoride, with the helium or other inert gas setforth, appears to have little, if any, effect on the percent yield ofchlorine pentafluoride. Chlorine trifium ride boils .at 11 C. GaseousClF is partially soluble in hydrogen fluoride. Liquid ClF is misciblewith HP. The lowest concentration of ClF with respect to HF set forthfrom line 32 as a carrier flowing over the surface of the 5 in the abovetable is a molar ratio of 550 (HF) :1(ClF liquid in the cell, thence upthrough the condenser 44 At lower concentrations of CIF it appears thatthe conand out through the by-pass 49. Thereafter, valve 46 iscentration of GE in the area adjacent the electrodes beopened, valve 50is close-d, branch line 57 leading to comes insutficient for the processof this invention'and storage is closed, purge outlet 60 is opened, ameasured fluorine and hydrogen are evolved with resultant exploamount ofchlorine trifluoride is added to the liquid HF sions. As theconcentration of ClF is increased to below in the cell through its inlet35, the electric power source a molar ratio of 1:1 (HF:CIF the vaporpressure of is energized, and the gaseous products (including C11 theCIF;, becomes so high that, with laboratory apparatus, pass into thecondenser 44. The condenser is cooled as a practical problem of removingthe greater volumes of with methanol at from about -l0 C. to 20 C. 15ClF gas presents itself. It is conjectured that in a comwhereby most ofthe hydrogen fluoride and chlorine trimercial process, recycling of theClF may not be objecfluoride vapors are refluxed back into the cell. Thecarrier tionable. Even at an HP to ClF molar ratio of about gas and thechlorine pentafluoride along with the other 15:1, with the use of thelaboratory apparatus described products of the electrolysis reactions,i.e. F ClF, and C1 hereinabove, a large amount of ClF was carried overby then flow through the HF absorber 48, and thence through the helium.A preferred concentration is about 40:1. the train of cold traps 54-55.The first cold trap 54 is Another facet of the matter of concentrationof the preferably cooled to 78 C. by envelopment in a bath components ofthe solution in the electrolytic cell is that (not shown) of dry ice andtrichlorethylene to collect of including other chemical compounds.Obviously, such ClF and most of the ClF The second cold trap 55 iscontaminants which contain oxygen and through compreferably cooled to-196 C. by a bath of liquid bination with fluorine produce undesirableproducts such nitrogen for collecting ClF and ClF Thereafter, purge as0P CIO F and ClO F, should be avoided. Bromine outlet 60 is closed, thetrain of traps is closed ofi from would interfere with the efficiency ofthe reaction in that the condenser and the cooling baths are removedwhereby it is more easily fluorinated than chlorine trifluoride. thecollected products in the traps pass as gases to stor- Silver fluorideand thalium fluoride, which are soluble age 57. in hydrogen fluoride,would lend conductivity to the The following table sets forthparticulars of operating solution in the cell; however, it is expectedthat silver conditions for several examples of the practice of the wouldplate out at the cathode. process of this invention using the apparatusdescribed Another variable to be considered as having an effect above:on the process of this invention is that of the pressure in CurrentVoltage Time Products other than 011% and 012 Test Mol. ratio in in inTemp. 0! No HF:MX:ClF.-l amps volts min. cell in C. ClF ClOz C1021?01031? SE, 1 16:0.39z1 0.5 4.5-4.4 60 -13 to 11 X X X 3.0 6.8-6.0 301620.4:1 0.72 4. 8-4.4 105 -14 X X X 5621.5:1 3.0 6.0-6.8 162 --10 X X X103:26:1 1.0 4.3-5.8 360 --15 X X X 550112.:1 1.0 5.0-5.8 1 -13 to 10 XX 25 1? i3 5 32% $23 X X X 52021351 213 815-710 90 X X X 6221.621 2.35.8-6.8 245 11 X X X 59:1.6z1 2.3 6.2-7.5 170 6 X X X In tests No. 1 theMX was KF. In the remaining tests the MX was NaF.

Percent yields of from about 15 to 20 were realized on a continuousoperational basis for all of the examples in the table. The term percentyield, as used herein, is calculated as 100 times the quotient of theactual yield of C11 in grams divided by the theoretical yield, withreference to formula No. 1 above. The theoretical yield in grams equalsthe number of coulombs passed multiplied by the gram equivalent weightof ClF i.e. 130.5 divided by 193,000 coulombs.

The efliciency of the process of this invention is affected by therelative concentrations of the components in the solution in theelectrolytic cell. In the examples of the above table, the molar ratioof hydrogen fluoride to the alkali metal halide is about 40 to 1. 'Whensuch ratio was increased to about 80 to l, the process of this inventionproceeded quantitatively, however a substantial increase in voltage wasrequired to attain the same amount of electric current, whereby theefliciency of the process was substantially reduced from a practicalstandpoint. Saturated solutions of the alkali metal halide in thehydrogen fluoride may be employed.

The relative concentration of chlorine trifluoride, with respect tohydrogen fluoride, within the limits hereinafter the electrolytic cell.For the examples in the above table, pressures of about 1 atmospherewere employed. Lower pressures lower the boiling point of ClF andthereby increase the amount of CIF carryover. An increase in pressuretends to decrease volatilization of the 01B, and HF and thereforepermits advantageous operation at higher temperatures.

Turning now to the factor of temperature as an operating condition forthe process of this invention, for the laboratory apparatus illustratedin the drawing and described above, a range of temperature of from 0 to-14 C. may be set as a limited range, at atmospheric pressure. Operationat below -14 C. would result in a batch process instead of theadvantageous continuous process by requiring periodic distillation forrecovery of ClF Increasing the solution temperature in the cell to above0 C. produces too much volatilization of HF and ClF for convenientlaboratory handling. It is important to notehowever, that for anelectrolytic process, an increase in temperature is advantageous becauseit increases the conductivity of the electrolyte whereby less voltageand hence less power is required for the same current density. Apreferred temperature is about 10 C.

The amount of current employed affects the efliciency of operation ofthe process of this invention. Generally,

the efficiency of the process increases with an increase of currentdensity (amps per unit area of effective electrode surface). With theabove deescribed laboratory ap aratus, currents of one-half to 3 ampswere employed. Use of currents below that range was not possible withthe detecting apparatus employed. At currents above 3 amps, explosionsoccurred and the increase in heat from increased current resulted intemperatures above the range of useable temperatures explained above forillustrated laboratory apparatus. The decomposition potential of the HFis about 3.9 volts at about C. It is possible, though not practical, bythe process of this invention to obtain ClF with voltages as low as 4.4volts. A preferred current is about 2 amps.

In the above described laboratory preparation of CIF according to theprocess of this invention, the ClF in storage 57 may be separated fromthe other components which were evolved from the cold traps 54 and 55 inany conventional separation operation, e.g. passing the contents ofstorage 57 through a low temperature fractional distillation column torecover the ClF It will be understood that it is intended to cover allchanges and modifications of the examples of the invention herein chosenfor the purpose of illustration which do not constitute departures fromthe spirit and scope of the invention.

Having described the invention, what is claimed is:

1. A process for synthesizing chlorine pentafiuoride comprising thesteps of passing an electric current between spaced electrodes in asolution of chlorine trifiuoride, hy-

drogen fluoride and an alkali metal halide whereby chlorinepentafluoride is evolved, and collecting the evolved chlorinepentafluoride.

2. The process of claim 1 in which said alkali metal halide is afluoride.

3. The process of claim 1 in which a percent yield of from about 15 toabout 20 is obtained when employing a laboratory apparatus comprising anelectrolytic cell having two plate electrodes spaced apart by a distanceof about one-half inch, each electrode being of about sq. cm. per plateface, the apparatus being operated under the following conditions: themolar ratio of HF: alkali metal halide is from about :1 to saturation ofthe alkali metal halide in HF the molar ratio of HF:ClF is from about550:1 to about 1:1; the pressure in the cell is about atmospheric; thetemperature of the solution in the cell is from about 0 to about -14 C.;and the current is from about one-half to 3 amps.

4. The process of claim 3, wherein said ratio of HF: alkali metal halideis about 40:1; said ratio of HF:CIF is about 40:1; said temperature isabout 10 C.: and said current is about 2 amps.

No references cited.

REUBEN EPSTEIN, Primary Examiner.

U.S. Cl. X.R. 1491

1. A PROCESS FOR SYNTHESIZING CHLORINE PENTAFLUORIDE COMPRISING THESTEPS OF PASSING AN ELECTRIC CURRENT BETWEEN SPACED ELECTRODES IN ASOLUTION OF CHLORINE TRIFLUORIDE, HYDROGEN FLUORIDE AND AN ALKALI METALHALIDE WHEREBY CHLORINE PENTAFLUORIDE IS EVOLVED, AND COLLECTING THEEVOLVED CHLORINE PENTAFLUORIDE.